Slip systems are typically used to anchor packers to the casing. A typical slip system comprises a cone, slips and a body. The cone is typically a cylindrical component which has a shallow angle cut on the outside diameter of one end. The slips are segments cut from a cylinder and have the same angle as the cone on the inside diameter, as well as sharp teeth on the outside face. The cone and slips slide over the body, which is also cylindrical. When the packer is set, the cone pushes against the slips through the shallow angle, causing them to move radially until the sharp teeth contact the casing. Load applied to the packer is transmitted to the cone, which causes the slips to bite deeper into the casing to prevent the packer from moving. Therefore, in most slip systems, a radial load is applied to the cone when the packer is loaded due to the angles cut on the cone and slips. If the load applied to the packer is great enough, the cone will collapse until the inside diameter of the cone contacts the outside diameter of the body. At times, the applied load can cause the body to collapse. The limitation of the amount of load a packer can hold is often determined by when the cone collapses onto the body, causing it to collapse. Thinner slip systems, because of their reduced cross-section, are less resistant to collapse from the applied radial load and hold less force than thicker systems. However, thick slip systems have a disadvantage of requiring additional space, which decreases the available bore size in the packer for a given casing size.
Another design which has been used in the past on packers is illustrated in FIGS. 1-3, as well as in U.S. Pat. No. 4,711,326. FIG. 1 is a perspective of a slip without the wickers, illustrating opposed beveled surfaces 10 and 12. Each of those surfaces has an elongated tab 14 and 16, respectively. Referring to FIGS. 2 and 3, the elongated tabs 14 and 16 ride in grooves 18 and 20. Grooves 18 and 20 are wider than the width of the tabs 14 and 16 to allow easy movement for guiding the slip 22 along the cone 24. As seen in FIG. 3, the cone 24 has opposed surfaces 26 and 28 which are disposed to engage the beveled surfaces 10 and 12 on slip 22 shown in FIG. 1. Thus, the extension of the tabs 14 and 16 into grooves 18 and 20 serves to guide the slip 22 with respect to cone 24, while at the same time the engagement of the beveled surfaces 10 and 12 on slip 22 to surfaces 26 and 28 of cone 24 acts to transfer the radial load from the casing through the slip 22 into the cone 24. Because of the beveled cut on surfaces 10 and 12, a near-circumferential component of the radial force applied to the slips 22 is communicated into the cone 24. This design has been used traditionally to hold forces from only one direction and in permanent installations. The present invention is more suitable for retrievable packers and systems which need to hold forces from both directions (bidirectional). The present invention retrieves because there is only one angle between the slip and cone instead of the combined angles in the prior art shown in FIGS. 1-3. This combined angle causes a wedging effect between the slips and cone which increases the retrieval force. Tests have shown that in some cases, the retrieval force is so high that the tails 15 are pulled off the ends of the slips due to a tensile failure at narrow region 17 (see FIG. 1). When this happens, the slips cannot be retrieved.
In the preferred embodiment, the present invention uses bidirectional slips which have a ramp angle on each end. The prior art slips of FIGS. 1-3 only have a ramp angle on one end. The prior art system of FIGS. 1-3 is not readily convertible to a bidirectional design, and even if it could be, it would still be very costly, highly complex, and not as reliable as the present invention.
These and other advantages of the present invention will be more readily understood by those skilled in the art from a review of the preferred embodiment described below.